The grate-kiln pelletizing process is a means of indurating iron ore into pellets suitable for transportation and subsequent use in blast-furnaces and steel-making. The iron ore fines are mixed with other materials such as dolomite and bentonite, and formed into round balls, which are then loaded onto a moving grate, where they are dried, preheated, and partially hardened. Final hardening takes place when the pellets are discharged from the grate into a large rotary kiln, where they are heated to 2400-2500 F by means of a large burner firing into a process air stream, with an excess of oxygen in the products of combustion. (In some cases oxidation of iron in the ore also provides heat input to the process.) The pellets are then cooled in a cooler by forcing a stream of ambient air through the pellets. The process air stream for the kiln is the hot air generated from cooling the pellets in the cooler combined with products of combustion from the kiln burner.
Since the heat transferred to the pellets in the grate and kiln sections is regenerated into the process air in the cooler, the process is very energy efficient, but the process requirement for large excess of oxygen in the kiln combined with the high air temperature in the air entering the kiln from the cooler also results in very high NOx. It would be valuable to be able to reduce the NOx generated by the kiln burner while still maintaining the high process efficiency by using the process air.
There are other similar processes that incorporate rotary kilns fired by a burner and supplied with a process air stream that has been pre-heated by cooling the product in a cooler. The invention is applicable to those processes as well.
Typical prior art grate-kiln pelletizing furnaces incorporate a large rotary kiln fired by one or two very high capacity (100 to 500 MMBtu/h) kiln burners which combust hydro-carbon fuels, usually natural gas, fuel oil, coal, or biomass, in an excess of high-temperature preheated air, to provide a high temperature (2400-2500 F) oxidizing environment which is needed to indurate iron ore pellets. The typical kiln is fired by a single large burner, with a very long high temperature flame. The large flame envelope results in a very large interface area between the flame and the high-temperature oxidant, and in long residence times. Thus, the large flame envelope, high preheat temperature, high flame temperature, and large excess of oxygen in the combustion zone all combine to generate very high NOx emissions.
The prior art design is very fuel efficient, in that the heat stored in the pellets is transferred to preheat air to temperatures as high as 2000 F; this air is then subsequently used for drying and heating the pellets, and as oxidant for the fuel needed to heat the process gases to the required temperature. The problem is that the factors that make the process fuel-efficient contribute strongly to the formation of NOx. Most of the strategies used by prior art low-NOx burners either do not work very well in the high temperature highly oxidizing environment, or have significant negative impacts on fuel efficiency.
The only other means available for NOx reduction have been after-treatment methods such as SCR, SNCR, and LO-TOX. These methods are either very expensive to implement, require significant additional energy input to the process, or are impractical to incorporate into the process.
In the context of maximizing fuel efficiency with unregulated emissions, the prior art arrangements make intuitive sense, as the highest temperature streams of recuperated cooling air are used in the highest temperature part of the process.
FIGS. 1, 2, and 3 show configurations typical of the prior art. In FIG. 1, hot indurated pellets are discharged from a kiln 10 into a cooler 12. A cooling air blower 14 blows a cooling air stream over the pellets in the cooler 12, cooling the pellets and heating the cooling air. The blower 14 is part of a gas flow apparatus that includes blowers, burners, ducts, flow control devices, controllers, and other known devices as needed, in a configuration that provides the heated process air and other reactants for the indurating process. The cooler 12 is typically segmented into stages or sections 20, with the cooling air leaving the sections 20 closest to the discharge end 22 of the kiln 10 hotter than the cooling air leaving the sections 20 farther from the discharge end 22 of the kiln 10. In the case of coolers in which the travel is rotational, such as annual coolers, the terms “closest” and “farther,” as used above, refer to the distance that the pellets have travelled along the path of rotation of the cooler 12, as opposed to the linear distance from the discharge end of the kiln 10.
In FIG. 1, air at approx. 2000 F air leaving the hottest section 20 of the cooler 12 passes through a combination of hood and duct structures 24 before entering the kiln 10. A kiln burner 26 typically fires one or more fuels such as natural gas, fuel-oil, coal, biomass, etc. into the discharge end 22 of the kiln 10. The kiln burner 26 is typically provided with a stream of combustion air which is much less than the amount required to completely combust the air. The process air from the cooler 12 includes a large excess of air compared to what is required to burn the fuel, so there are typically oxygen levels from 10% to 16% in the process air leaving the kiln 10 after fully combusting all of the fuel.
The process gases leaving the kiln 10 pass through one or more ducts 30 to the final preheat section 34 of a traveling grate 36. Dried and partially hardened pellets discharge from the grate 36 into the kiln 10 where the indurating process is completed. The process gases at perhaps approximately 2400 F are induced by a process gas blower 38 to flow through the pellet bed on the grate 36, preheating the pellets; in doing so, the process gases are cooled to perhaps 600 F before entering the process gas blower 38. The process gas blower 38 then discharges the process gases through ducts 40 into the drying section 42 of the grate 36. The pellets at approximately ambient temperature enter the drying section 42 at the feed end 44 of the grate 36. In drying the pellets, the process gases are further cooled to a temperature typically between 200 and 400 F, before being discharged to atmosphere through an induced draft fan 46 and stack 48. It is typical for the exhaust to also be processed by means of equipment such as cyclone separators, electro-static precipitators, or baghouses (none of which are shown) to remove particulates before being discharged into the stack.
Typically, there are also one or more intermediate stages 50 of drying and/or preheat sections between the first drying section 42 and the final preheat section 34. In one typical configuration (FIG. 1), hot air at perhaps 1300 F from an intermediate section 20 of the cooler 12 is directed through a duct 51 and further heated by an air heater 52 to about 1500 F before being ducted to one of the intermediate sections 50 of the grate 36. The air heater 52 is fired by a burner 54 using a fuel (typically natural gas, propane, or fuel oil) to provide the heat necessary for raising the temperature of the hot air to the required level. The process gases from the air heater 52 are then drawn through one or more intermediate drying/preheat sections 50, sometimes in a combination of updraft and downdraft configurations (not shown), before being processed through gas clean-up equipment (not shown) as described earlier and then being exhausted to atmosphere.
A slightly different known configuration is shown in FIG. 2. The difference between FIG. 2 and FIG. 1 is that in FIG. 2, the process gas from the intermediate section 20 of the cooler 12 does not pass through the air heater 52. Instead, the air heater 52 incorporates a stream of dilution air from a dilution air blower 56 (or alternatively from a combined air heater combustion air and dilution air blower—this alternative not shown) to create a hot gas stream of perhaps 2000 F that mixes with the incoming air stream from the intermediate stage 20 of the cooler 12 to produce a combined process gas stream at 1500 F, with a higher total mass flow rate, which is then directed to the grate as in FIG. 1.
Another known configuration in the prior art is shown in FIG. 3. In FIG. 3, preheat burners 60 are installed in the roof or sides of the preheat and/or drying sections of the grate 36. The preheat burners 60 may be used instead of the air heater 52 shown in FIGS. 1 and 2 or in addition to the air heater 52.
In each prior art arrangement of FIGS. 1, 2, and 3, process air of approximately 800 F from the final (coolest) stages 20 of the cooler 12 may be directed through ducts 62 to other parts of the plant (such as for grinding), or may be exhausted to the atmosphere through a stack 64 at approximately 300 F. The elements that differ between the three prior art configurations are sometimes used in combination with each other, e.g. some configurations have both preheat burners 60 and an air heater 52.